1. Field of the Invention
The present invention relates to a battery protection circuit for preventing a secondary cell from being overcharged and overdischarged.
2. Description of the Related Art
Battery packs composed of a plurality of series-connected cells are protected from damage either by a battery charger which detects overcharging or overdischarging with respect to a total voltage across the series-connected cell, or by turning on and off a load connected to the battery pack.
Generally battery chargers or battery loads, rather than batteries themselves, have a battery protecting function. When abnormal overcharging is caused by a battery charger or an overcurrent flows through a battery load, a thermostat or the like in contact with the battery or battery pack detects the temperature and turns off the power supply to the charges or disconnects the load.
Another known battery protection circuit is disclosed in Japanese laid-open patent publication No. 04-75430 published Mar. 10, 1992 which was filed by Asahi Chemical Industry Co., Ltd.
According to the disclosed arrangement, a chargeable power supply device having a boost-chargeable secondary cell employs a device having a parasitic diode in the charging and discharging circuits to prevent overdischarging and overcharging.
More specifically, when the voltage across a cell drops below a certain voltage at the time it is discharged, a controller produces an output voltage to render a MOSFET non-conductive, thus cutting off the discharging circuit for the cell. When the voltage across the cell is restored by being charged through the parasitic diode of the MOSFET, the cell is charged in a normal mode with a low loss. Similarly, when the voltage across the cell rises above a certain voltage at the time it is charged, the controller produces an output voltage to render a MOSFET nonconductive, thus cutting off the charging circuit for the cell. When the voltage across the cell is restored by being discharged through the parasitic diode of the MOSFET, the cell is discharged in a normal mode with a low loss.
With the above process, however, if a battery is composed of a plurality of cells which have different characteristics, some of the cells tend to be overcharged or overdischarged. The above process cannot be used with respect to those cells which are damaged when overcharged and overdischarged.
There have also been proposed battery protection circuits as shown in prior inventions disclosed in Japanese patent application No. 3-213019 filed Jul. 31, 1991 and Japanese laid-open patent publication No. 5-49181 published Feb. 26, 1993, both filed by the same applicant as the present application. According to these prior inventions, while series-connected secondary cells are being repeatedly charged and discharged, even if the capacities of the cells are unequal due to the differences between the individual cells, the capacities of the cells can be brought back into equilibrium by a cell capacity balancing circuit in the charging and discharging circuit thereby preventing the cells from being overcharged and overdischarged.
The cell capacity balancing circuit is basically composed of a circuit arrangement for protecting the cells which comprises, for battery protection, a circuit for detecting overcharging, a circuit for detecting turning-off and overflowing of a charging current, a circuit for detecting overdischarging, and a circuit for detecting turning-off of a discharging current, hysteresis, and an overcurrent.
The battery protection circuits disclosed in the above prior publications and application are however disadvantageous in that even though overdischarging and overcharging of cells are detected, a current flows through the circuits after overdischarging is detected, and hence current consumption occurs at all times.
If a battery is composed of a plurality of cells and the cells have different characteristics, then only certain cells are liable to be overcharged or overdischarged. Such overcharging and overdischarging affect the performance of the cells.